A baby has been born using three people's DNA for the first time in the UK.
Most of the child's DNA comes from their two parents but about 0.1% has come from a third person - another woman.
The Human Fertilisation and Embryology Authority (HFEA) said “less than five" babies have been born in the UK this way, but no further details have been released, to protect their identity.
The pioneering technique is designed to prevent children being born with devastating mitochondrial diseases.
The incurable diseases can be fatal within days or even hours of birth. Some families have lost multiple children.
In children, symptoms can include poor growth, poor muscle tone, weakness, failure to thrive, spasms and a slow-down in progress.
The new technique - mitochondrial donation treatment, which is a modified form of IVF - is seen as their only chance of having a healthy child.
The DNA from the second woman only affects the mitochondria, and does not affect other key traits in the child such as appearance. This means the third person is not counted as a third ‘parent’.
Mitochondria are the compartments inside nearly every cell of the body that convert food into energy.
Defective mitochondria fail to fuel the body and lead to brain damage, muscle wasting, heart failure and blindness.
The latest findings were first reported by the Guardian newspaper following a freedom of information request.
Britain became the first country in the world to formally allow mitochondrial replacement therapy (MRT) when the HFEA gave a cautious green light to the procedure in 2017.
In 2018, fertility doctors at the Newcastle Fertility Centre at Life were given permission by HFEA to give two women the treatment.
The first baby born by this technique in the world was to a Jordanian family having treatment in the US in 2016.
Peter Thompson, chief executive of the HFEA, said: “Mitochondrial donation treatment offers families with severe inherited mitochondrial illness the possibility of a healthy child.
“The UK was the first country in the world to allow mitochondrial donation treatment within a regulatory environment.
“The HFEA oversee a robust framework which ensures that mitochondrial donation is provided in a safe and ethical manner.
“All applications for treatment are assessed on an individual basis against the tests set out in the law and only after independent advice from experts.
“These are still early days for mitochondrial donation treatment and the HFEA continues to review clinical and scientific developments."
Sarah Norcross, director of the Progress Education Trust, said UK laws relating to the treatment were “passed only after many years of careful research, assessment and deliberation".
She added: “Even then, it was decided that use of this technology would be permitted by the regulator only on a case-by-case basis.”
